The invention relates to the field of quadrature modulation communication systems. More particularly, the present invention relates to vestigial sideband modulation communications systems using cross coupled independent data stream modulated on a common carrier in quadrature.
The rapid, worldwide expansion of communications services underscores the importance of bandwidth conservation. With increased demands for cellular and personal communications services within a finite radio frequency spectrum, there is an ever-increasing contention for bandwidth. Cellular services are growing at a geometric rate. Microcellular sites are being advocated to handle the increased demand through localized frequency reuse, and hundreds of low earth orbit and medium earth orbit satellites will support the increasing demand for bandwidth over the next decade. In digital video communications, high definition television (HDTV) transmits at 21.5 Mbits/s with a greatly improved picture quality that must be compatible with the existing 6.0 MHz channel bandwidth allocation. This requires a bandwidth efficiency of greater than 3.0 bits/s/Hz. Additionally, data throughputs in communications have also been increasing at an exponential rate. Existing bandwidth allocations are typically shared among different services. A review of the current frequency allocations reveals that the majority of bands exhibit sharing of multiple services, such as, fixed and mobile satellite services and earth exploration satellites. A natural consequence of this sharing is increasing interference. With the bandwidth being a finite resource, there are increasing demands for this finite bandwidth resources creating a need to develop general purpose practical bandwidth efficiency communication techniques.
Digital data has been transmitted using double sideband (DSB) or quadrature double sideband (QDSB) techniques. Occasionally, single sideband (SSB) formats have been used, and more recently two vestigial sideband (VSB) formats have been selected as the standards for off the air and cable HDTV. DSB signaling is the simplest and most straight forward means to transmit analog or digital information on a carrier, such as, when using AM and FM methods. SSB is employed when the bandwidth is at a premium, such as, when multiplexing terrestrial telephone channels. VSB is used when requiring a controlled component of energy at the carrier frequency, such as, in TV and HDTV communications.
One of the most useful ways to assess bandwidth efficiency is to make use of the Shannon channel capacity bound that provides an upper limit on the signaling rate Rs for error free transmission over an arbitrary channel. Modern digital modulation techniques are compared to the Shannon channel capacity bound to provide a performance overview. When the maximum signaling rate is normalized by the required transmission bandwidth, a measure of the bandwidth efficiency of the modulation method is obtained in units of bits/s/Hz. This normalized performance benchmark is known for many of the widely used modulation formats. Unfiltered digital data typically has a sin(x)/(x) frequency response with significant sidelobe content over a bandwidth wider than the data symbol rate. The Nyquist technique is used to transmit digital data within a limited bandwidth without intersymbol interference. Intersymbol interference (ISI) is eliminated when the response magnitude through a transmission channel has vestigial symmetry about the half amplitude point that occurs at a frequency equal to half the symbol rate with the communication channel providing a linear phase response. When the magnitude response of the channel transmission function has vestigial symmetry about the half amplitude point that occurs at a frequency equal to half the symbol rate, and when the transmission function has linear phase, data can be communicated without ISI. The bandwidth efficiency has been calculated assuming transmission at the minimum Nyquist bandwidth Rs/2. A Eb/No scale is used to derive a bit error ratio (BER), for example, 10xe2x88x926, during data communications.
The single sideband and quadrature single sideband (QSSB) modulation format data points have exactly twice the bandwidth efficiency of the corresponding double sideband counterparts where the effect of quadrature channel crosstalk can be rendered negligible. The quadrature channel crosstalk is inherent in QSSB transmission in which independent data is placed on quadrature carriers. The crosstalk degrades performance and has been a major problem in QSSB communications. The DSB techniques diverge from the bound as the number of bits/s/Hz or bandwidth efficiency is increased, whereas ideally transmitted QSSB formats run parallel to the bound. This divergence is due to the redundancy in transmitting two sideband replicas. As the bandwidth efficiency of the channel is increased, QSSB potentially offers a progressively larger advantage over DSB transmission. In particular, when a six bit/s/Hz efficiency is needed, a conventional phase shift keying (PSK) may be used, such as 64-PSK. The DSB scheme would be required as compared to an 8-PSK QSSB format. The DSB scheme requires 18 dB more signal power to achieve the same BER. In general, the number of signal levels needed with DSB techniques is the square of that required with an equivalent QSSB format. These large discrepancies in signal to noise ratio (SNR) and number of signal levels leave considerable margin for non-ideal SSB signaling due to crosstalk. SSB uses half the bandwidth of conventional DSB yielding twice the bandwidth efficiency. Because of the sharp cutoff characteristics at one of the SSB band edges, vestigial sideband method is often used to realize a more gradual rolloff. The VSB method is not as bandwidth efficient as the SSB method, but generally leads to a more practical solution with controlled crosstalk. Conventional VSB filtering uses inphase and quadrature arm filters in both the transmit modulators and receive demodulators. The data stream is reinforced in the receiver by transmitting the data stream through both quadrature and inphase channels. The quadrature arm odd responses combine to yield a even response. The VSB is slightly less bandwidth efficient than SSB, but is more controlled and easier to implement. VSB or SSB frequency spectra transform to inphase and quadrature impulse response with even and odd time symmetry, respectively.
In practice, during VSB modem transmit and receive filtering modeling, matched filtering is employed so that the square root of the Nyquist frequency response is apportioned equally to the transmitter and receiver as opposed to a full response. In general, impulse responses with equally spaced axis crossings will only occur after passing through matched sets of transmit and receive filters. The critical filtering for a Nyquist band limited VSB transmitters and receivers is employed in HDTV. In the HDTV system, a single data channel is communicated through both inphase and quadrature (IandQ) channels respectively having a hi arm filter and a hq arm filter in both of the transmitter and receiver for communicating i(t) and iH(t) signals. The transmit square root hi and hq arm filters have an even and odd impulse response relationship but neither has equally spaced axis crossings. The H subscript is used to denote the odd impulse responses that are similar to the Hilbert transform of the even impulse responses. The Hilbert transform j*sgn(f) has an abrupt 90xc2x0 phase transition in the frequency domain. The Hilbert transform is used to realize the precipitous sideband rejection in SSB via the phase shift generation method. The sideband rejection in VSB is more gradual. Sideband rejection is typically realized through a combination of IandQ channel amplitude mismatch in conjunction with a Hilbert phase shift discontinuity and hence similar to the Hilbert transform.
In the receiver, both the IandQ filter output responses are approximately the same with each having even time symmetry. The I-channel response is related to the Q-channel response. The Q-channel response is approximately the same at the I-Channel response because it is roughly the cascade of two Hilbert transform 90xc2x0 phase shifted versions of the I-channel response that which is merely an inverted version of the I-channel response. The Q-channel output then is summed with the I-channel to improve the detection SNR by 3.0 dB. There is no ISI problem when Nyquist filtering is used in conjunction with VSB data transmission as is evidenced by I-channel responses in a 25% raised cosine VSB eye diagram.
Vestigial sideband is defined such that when its spectrum is downconverted to baseband, the inner transition regions of its positive and negative frequency image bands overlap and are complementary so as to sum to unity with proper phasing. VSB is a good compromise between DSB and SSB because VSB approaches SSB in bandwidth efficiency, but does not require an infinitely sharp transition band. HDTV will be transmitted digitally using trellis coded 8-ary VSB and 16-ary VSB formats, for terrestrial and cable distribution, respectively. These VSB formats require 8-ary and 16-ary amplitude levels in their baseband modulating waveforms. To facilitate VSB signaling, a common digital data stream modulates two quadrature carriers where the impulse response pairs are orthogonal and correlated. Quadrature SSB (QSSB) and QVSB are more complex than SSB and VSB because each of the inphase (I) and quadrature (Q) baseband modulating channels contain the superposition of an independent pair of data streams having interference and crosstalk that must be controlled. The advantage of QSSB/QVSB over SSB/VSB is a doubling of the information carrying capacity. The disadvantages are greater implementation complexity, and a typically reduced noise margin due to crosstalk.
There are fundamentally two methods of generating SSB/VSB, that is, the quadrature phase shift method and sideband filtering method. In this study, the phase shift method is favored over the filtering approach, because precise control over the modulating waveshapes will be necessary; and this precision is best achieved with digital signal processing techniques. The phase shift approach is shown analytically for an SSB modulator output, s(t) by an SSB modulator output equation s(t)=i(t)cos(xcfx89ct)xc2x1ĩ(t)sin(xcfx89ct), where the baseband message waveform, i(t) and the Hilbert transform ĩ(t) modulate quadrature carriers. The minus sign on the Hilbert component yields upper sideband (USB), whereas a plus sign gives lower sideband (LSB). VSB can also be represented in this manner, but the inphase and quadrature components are not strictly Hilbert transforms.
To conserve bandwidth using SSB/VSB modem baseband filtering modeling, digital modulation techniques are filtered prior to transmission. To maximize the detection SNR, the receive filtering is matched to the transmit filter. The critical baseband filtering for a band limited SSB/VSB modem uses a single data stream with single arm filtering for VSB modulation. The modulated output signal is generated according to modulator output equation. To facilitate SSB transmission, the inphase filters with the i subscripts and the quadrature filters with the q subscripts must be a Hilbert transform pair hq and Hq, such that, hq(t)=(1/xcfx80t)*hi(t) and Hq(f)=jsgn(f)Hi(f). The symbol * is the convolution operator, and (t) is the time domain variable and (f) is the frequency domain variable. The Hilbert transform pair hi and hq are orthogonal by definition, and with a perfectly balanced structure, complete cancellation of one of the sidebands results. When hi(t) has even symmetry, hq(t) would have odd. From the frequency response definition in H(f), the cascaded response of any two quadrature filters is the negative of the inphase filter responses, for example hq*hq=xe2x88x92hi. Because the noises in the I/Q detection arms are uncorrelated, and the signal components are perfectly negatively correlated, combining the IandQ filtered outputs yields a 3.0 dB improvement in the detection SNR. The term {tilde over (h)}(t) is the Hilbert transform h(t). The double tilde term {tilde over ({tilde over (h)})}(t) is the Hilbert transform of {tilde over (h)}(t). Subtracting the double tilde impulse response {tilde over ({tilde over (h)})}i from the inphase counterpart acts as constructive interference where hq(t)={tilde over (h)}i(t) and hi(t)={tilde over (h)}q(t) and {tilde over ({tilde over (h)})}i(t)=xe2x88x92hi(t). For VSB transmission, the hi and hq filter pairs are not strictly Hilbert transforms of one another, but have vestigial symmetry about the half power points in the frequency domain. This type of VSB modem is used in HDTV, where i(t) has eight or sixteen detection levels.
For memoryless Nyquist filtering, the Nyquist family of filters are evaluated for applicability in achieving bandwidth efficient transmission with minimal degradation in SNR performance due to ISI. Ideal rectangular and raised cosine filtering have been used for Nyquist filtering. Nyquist impulse responses are sinc based waveshapes with even time symmetry and equally spaced zero or axis crossings at integer multiples of the data symbol time. As a result, responses from adjacent data symbols do not interfere at the detection sampling instants. The impulse responses with equally spaced axis crossings are realized when the frequency response has vestigial symmetry about the half amplitude transmission points. The most concentrated distribution of signal bandwidth in the frequency domain is the ideal rectangular spectrum using ideal rectangular filtering. The magnitude for an SSB version of the ideal rectangular spectrum for the minimum Nyquist bandwidth Rs/2 can be considered on a frequency axis normalized by the data symbol rate. The analytic signal is used so that SSB frequency response is at baseband. The ideal rectangular spectrum represents the sharpest cutoff extreme of the Nyquist filtering including the raised cosine filtering. The inphase impulse response corresponding to the SSB rectangular spectrum is the sinc function, and the quadrature impulse response is a raised cosine with both decaying at 1/t. Because the two impulses responses are Hilbert transform pairs, the quadrature term hq(t) will have odd symmetry because the quadrature term is odd and equivalent to 1/xcfx80t convolved with the even sinc function. The inphase and quadrature transform pair is given by hi(t)=sin(xcfx80Rst)/xcfx80Rst and hq=(1xe2x88x92cos(xcfx80Rst))/xcfx80Rst.
When the SSB spectrum is band limited to half the data symbol rate, the corresponding inphase impulse response will have equally spaced axis crossings at integer multiples of the symbol time Ts and the quadrature impulse response will be zero at xc2x1even multiples of the data symbol time. The quadrature impulse response has a 1/xcfx80t symmetry. In a QSSB scheme, the quadrature impulse response component from one channel will overlay the inphase component of the other channel. Hence, in the case of an ideal rectangular SSB quadrature impulse response pair, the quadrature component will contribute ISI at xc2x1odd multiples of the data symbol time. Although the envelope of the ISI only falls off as 1/t the ISI dispersion does not diverge because for random data sequences with half of the ISI positive and half negative resulting in significant cancellation. When the bandwidth of the rectangular SSB spectrum is doubled to Rs, the resulting quadrature impulse response pairs are zero at all adjacent symbol integer multiples where there will be no ISI at the detection sampling points. However, to achieve this perfect isolation, the same bandwidth as DSB is required.
A widely used Nyquist filter realization is the raised cosine, which has a sinusoidally shaped transition band. The frequency response for a raised cosine filter is defined by H(f). The raised cosine H(f) equation is a frequency response equation that defines the VSB magnitude response. The corresponding impulse response is defined as h(t). The rolloff factor is 0 less than r less than 1 and the half amplitude frequency is fh=Rs/2. A closed form expression for the time domain Hilbert transform of the impulse response has not yet been found. The H(f) frequency response and h(t) time domain impulse response equations are used to model the raised cosine filter.       H    ⁡          (      f      )        =      {                                                                                                                                  1                      2                                        ⁢                                          {                                              1                        -                                                  sin                          ⁡                                                      [                                                                                          π                                ⁡                                                                  (                                                                                                            f                                      /                                                                              f                                        h                                                                                                              -                                    1                                                                    )                                                                                                                            2                                ⁢                                r                                                                                      ]                                                                                              }                                                                      ,                                            0                ,                                  xe2x80x83                                ⁢                                  f                  ≥                                                            (                                              1                        +                        r                                            )                                        ⁢                                          f                      h                                                        ≡                                      f                    0                                                                                      1              ,                              xe2x80x83                            ⁢                              f                ≤                                                      (                                          1                      -                      r                                        )                                    ⁢                                      f                    h                                                  ≡                                  f                  1                                                              ⁢                      xe2x80x83                    ⁢                      f            1                          ≤        f        ≤                              f            h                    ⁢                      
                    ⁢                      h            ⁡                          (              t              )                                          =                                    sin            ⁡                          (                              π                ⁢                                  xe2x80x83                                ⁢                                  R                  s                                ⁢                t                            )                                ·                      cos            ⁡                          (                              π                ⁢                                  xe2x80x83                                ⁢                                  R                  s                                ⁢                t                            )                                                            (                          π              ⁢                              xe2x80x83                            ⁢                              R                s                            ⁢              t                        )                    ·                      [                          1              -                                                (                                      2                    ⁢                                          rR                      s                                        ⁢                    t                                    )                                2                                      ]                              
A Nyquist frequency response is known for a 20% raised cosine filter in a VSB channel. In practice, this filter can be closely approximated, but not realized exactly because of the perfectly flat passband and stopband. In addition, the stopband also has infinite attenuation. The corresponding impulse response pair for the 20% square root raised cosine VSB response can be generated by means of an FFT, and the impulse responses are very similar to the ideal rectangular filter pair except ideal rectangular filter pair have more ringing due to an abrupt transition band. The even response has equally spaced axis crossings, and the odd response has zeros at xc2x1odd multiples of the data symbol interval.
Employing conventional raised cosine family filtering for QSSB or QVSB transmission would result in crosstalk that reduces the intersignal distances thereby degrading the BER performance. Partial response signaling has been used for SSB transmission. However, partial response signaling has not been extended to QSSB. The well known class-4 (1-D2) system has no DC content and is characterized by a half sine wave magnitude response of a total bandwidth Rs/2. The (1-D2) moniker implies that for each data symbol input, the PR filter outputs the difference of a data modulated sinc pulse with a two symbol delayed version. For this case, the Hilbert response does not have equally spaced axis crossings at xc2x1even multiples of the data symbol time. In analyzing the band limiting pulse shapes for Nyquist equally spaced axis crossings, the cross correlation of the I/Q filter pairs should be zero at the detection sampling instants. In a typical modem, matched filters that are the square root of the Nyquist frequency transmittance function, are placed in the modulator and demodulator. The transmit output that only passes through the square root impulse response will generally not have equally spaced axis crossings resulting in ISI.
A restricted type of QVSB signaling has previously been disclosed in 1985. The QVSB system had two IandQ inphase and quadrature data channels modulated in quadrature by a carrier in the receiver and demodulated in quadrature in the receiver. There were no arm filters in the IandQ channels in the transmitter or receiver. At the output of the QVSB transmitter and at the input of the receiver were disposed bandpass raise cosine filters for VSB communication. The QVSB system operated only for very soft rolloff spectra of restricted bandwidth efficiency range with substantial degradations in signal to noise ratio (SNR) due to crosstalk. The QVSB data transmission used Nyquist filters from the raised cosine filtering to band limit the signal. Nyquist filtering is widely used to eliminate intersymbol interference in conventional digital data transmission schemes. However, in the QVSB system, Nyquist filtering exhibits quadrature crosstalk and ISI in both channels. The QVSB system has crosstalk between the inphase and quadrature (IandQ) channels in a controlled form similar to intersymbol interference in partial response systems. The QVSB system could use a maximum likelihood sequence estimator (MLSE) to remove the ISI based on a.Viterbi algorithm. The QVSB system could employ digital data feedback in the synchronization loops. These techniques are taught in U.S. Pat. No. 4,419,759, entitled Concurrent Carrier and Clock Synchronization for Data Transmission Systems, and U.S. Pat. No. 4,472,817, entitled Non-PLL Concurrent Carrier and Clock Synchronization. The QVSB system can behave like partial response systems where preceding could be used to avoid error propagation. However, the QVSB system precoder did not exploit the correlation information in the received samples. Consequently, the Viterbi probabilistic MLSE decoder showed a marked improvement over precoding. The QVSB system achieved a bandwidth efficiency of 2.3 bits/s/Hz for a 75% raised cosine rolloff passband. This is double-the rate of 1.14 bits/s/Hz for QPSK transmission with a corresponding rolloff passband. A digital SNR Eb/No penalty of approximately 2.1 dB at a bit error ratio (BER) of 10xe2x88x925 was experienced as a result of the crosstalk. At a bandwidth efficiency of 3.0 Bits/s/Hz, the BER performance degraded by about an additional 5.0 dB due to the increased crosstalk.
Nyquist filtering during VSB data transmission for QVSB signaling can be analyzed using an eye diagram. An eye diagram is an overlay of the time response for all possible data sequences. The eye diagram highlights the effects of ISI. For the case of binary data, the Nyquist filtered waveforms that make up the eye diagram are typically bipolar. Hence, a threshold is set at zero and samples are taken in the center, at the maximum eye opening. Sample values above zero are detected as positive ones and samples below zero are detected as negative ones, that is, digital ones and zeros. Nyquist filtering does eliminate ISI at integer symbol time multiples. Hence, it is known that digital data may be transmitted without ISI when the channel filter response satisfies the Nyquist criterion. The best linear channel detection performance is obtained by matching the transmit and receive filter responses. The best known Nyquist filters are the raised cosine filters. For example, a VSB full raised cosine frequency response with a 25% rolloff rate would have corresponding inphase and quadrature impulse responses. These impulse responses correspond to the overall Nyquist channel response when a single data one is transmitted. Opposite polarity impulse responses would be used when a data zero is transmitted. To facilitate VSB, complementary impulse responses with even and odd time symmetry are needed in the quadrature channels. The impulse response horizontal axis marks are spaced such that adjacent symbol responses are centered at integer symbol time multiples. The tails from adjacent symbol impulse responses will overlap. However, for the inphase impulse response, there is no ISI at integer symbol time multiples. Therefore, data sequences can be symbol by symbol detected without any degradation in SNR performance. The quadrature impulse response has ISI only at odd symbol time multiples. The restricted QVSB system achieved a good BER performance using a 100% raised cosine filter. The performance for the 75% and 50% cases was substantially degraded, and solutions do not converge below 50% rolloff. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide bandwidth efficient communications using quadrature vestigial sideband signaling.
Another object of the invention is to generate bandwidth efficient IandQ channel waveshapes that exhibit minimal intersymbol interference and crosstalk.
Yet another object of the invention is to generate bandwidth efficient IandQ channel waveshapes that exhibit minimal intersymbol interference and crosstalk with reduced bit error rates.
The present invention is a method for transmitting digital data in a bandwidth efficient manner using a quadrature vestigial sideband (QVSB) signaling. The method can be used in data communication systems. The QVSB method may double the capacity of comparable conventional formats by placing overlapping independent data on each of two carriers in phase quadrature using cross coupled arm filters. The data overlap is necessary to achieve QVSB spectral occupancy.
The method eliminates as much of the crosstalk as desired in progressive steps. The method is realized by modulating transmit and demodulating receive hardware architectures, the later of which preferably including a quadrature crosstalk maximum likelihood sequence estimator (QCMLSE) specifically designed to support QVSB signaling within I and Q channel crosstalk. Using various combinations of filtering and higher level signaling constellations, the method can provide as high a bandwidth efficiency within signal processing technology permits with relatively little degradation in the signal to noise ratio (SNR).
A normalized channel capacity versus SNR for the QVSB implementation can be derived from models of the QVSB structure within a linear additive white Gaussian noise channel at perfect synchronization. Over a very broad range of raised cosine filter rolloffs, 4-ary QVSB achieves the same capacity as conventional 16-ary quadrature double sideband (QDSB), with up to 2.0 dB less required SNR at a BER=10xe2x88x925, and up to 5.5 dB less required SNR for 16-ary QVSB. The implementation works down to 0% rolloff that is equivalent to the ideal rectangular brick-wall filter response. In addition to the raised-cosine family, jump filters can be used to yield better capacity performance improvements. The performance is better at higher BERs, such as 10xe2x88x924 and 10xe2x88x923. The method can be augmented by forward error correction coding.
Operation with a 4-ary rectangular constellation over the complete range of Nyquist spectral rolloff characteristics has been achieved up to and including the 25% raised cosine response with graceful SNR degradation. Thus, the method is robust with greater bandwidth efficiency that can be realized via sharper rolloff. M-ary QVSB signaling achieves twice the capacity of M-ary QDSB signaling that is equivalent to the capacity of M2-ary QDSB. In addition, M-ary QVSB attains the bandwidth efficiency with several dB less SNR than required for QDSB. Due to the percent rolloff definition for raised cosine filters, the same percent rolloff for QVSB and QDSB results in a transition band that is half as wide for QVSB and hence the factor of two in the bandwidth efficiency. QVSB spectral shaping enables all significant intersymbol interference (ISI) beyond the adjacent symbols of the crosstalk to be eliminated. Hence, the complexity of the QCMLSE decoder, that increases geometrically versus the number of additional ISI points, is reduced.
The method achieves more bandwidth efficient data transmission using QVSB signaling. Modulator and demodulator hardware structures implementing the method enable improved bandwidth efficient communications. These modem structures include SNR efficient synchronization loops that will substantially outperform brute force squaring circuitry. The method preferably relies upon transmit and receive data filtering, specialized QVSB spectra generation, the QCMLSE Viterbi decoding, and a coherently aiding demodulator synchronization loop.
The symbol integer spaced zeros in the quadrature impulse response as well as the inphase response are preferably realized by jump filtering. The ISI removal at 6 (xc2x13) symbols can be realized through multiplication of the impulse responses by a time domain cosine waveform with a 6xc2x13 symbol period. When the time domain cosine multiplication is performed on a sin(x)/(x) response, the corresponding ideal rectangular single jump spectra is shifted up and down resulting in the double jump spectrum. The ideal rectangular bandwidth is expanded by ⅙th. Similarly, the ISI at xc2x15, xc2x17 symbols can be eliminated by further multiplication and spectral shifting, resulting in the quadruple jump and octal jump spectra, respectively. A less complex alternative, that also results in greatly improved ISI, can be realized by smoothing the transitions of the double jump spectrum. The ISI at xc2x11 symbols can not be removed because a doubling of the bandwidth would result. Hence, whenever a data pulse is transmitted, the pulse will be subjected to and dominated by controlled ISI from the adjacent symbols in the opposing quadrature channel and this ISI will be of approximate relative magnitude xc2x10.5. The QCMSLE provides for effective Viterbi decoding that minimizes the effect of the controlled ISI for improved bandwidth efficient communications. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.